Recently, an X-ray CT-apparatus has been broadly used in the field of medical diagnosis. For an X-ray detector in a conventional X-ray CT-apparatus, an ionization chamber which is filled with a high-pressure xenon gas has been, in general, used. However, a detector of this ionization-chamber type had such problems that the detector is large and heavy in itself; that an ionization current or a signal is not sufficiently large; that the absorption of X-rays is small; and that a residual ion current lies. Thus, with the X-ray detector of ionization-chamber type, due to these problems, it was difficult to develop an X-ray CT-apparatus of high performance having such characteristics that a signal/noise ratio (S/N ratio) is excellent, and diagnostic performance, spatial resolution and time resolution or high-speed scanning property are high.
Then, a solid-state X-ray detector in which a mono-crystalline Bi.sub.4 Ge.sub.3 O.sub.12 scintillator or a CdWO.sub.4 phosphor is used, has been developed, and is partially used in practice. However, the solid-state X-ray detector using the CdWO.sub.4 phosphor is smaller in a signal than a xenon-gas detector, and can not provide a sufficiently satisfactory S/N ratio. This shows a limitation in the intrinsic luminescence efficiency which is possessed by the CdWO.sub.4 phosphor.
Accordingly, as a phosphor for the solid-state X-ray detector, it has been required to employ a phosphor which is rather larger in radiation absorbing performance than the CdWO.sub.4 phosphor, and is high in conversion efficiency of from radiation into light, and is speedy in the response of luminescence to radiation, for example, a rare earth oxysulfide phosphor which is represented by Gd.sub.2 O.sub.2 S:Pr or the like.
However, it has been clarified that this rare earth oxysulfide phosphor produces the phenomenon of a slightly long afterglow, that is, that a feeble luminescence is left even when 1/1000 to 1/100 seconds have been passed away after X-ray irradiation was cut off. Thereby, it has been clarified that the above-mentioned rare earth oxysulfide phosphor can not be employed for a certain radiation detector for an X-ray CT, in which an especially feeble afterglow is required, and a complicated detector-apparatus for removing the effect of the afterglow of luminescence from the phosphor is required, if the rare earth oxysulfide phosphor is employed. Thus, if a phosphor leaving a feeble afterglow could be obtained, the phosphor could be employed also for the above-mentioned radiation detector for an X-ray CT.
As countermeasures therefor, it has been proposed to use, for example, a rare earth oxysulfide phosphor such as Gd.sub.2 O.sub.2 S:Pr phosphor to which Ce is added as disclosed in Japanese Patent KOKAI (Laid-Open) No.6-145655, and a phosphor to which a trace halogen element is added as disclosed in Japanese Patent publication No.60-4856 so as to shorten the time of afterglow.
According to experiments by the present inventors, it has been found that when Ce is added to the rare earth oxysulfide phosphor in a certain amount or more, the phosphor is colored into a yellowish color in itself, and the luminescence output thereof is decreased in itself. Therefore, it has been impossible to sufficiently exert the characteristics of the rare earth oxysulfide phosphor, and accordingly, and X-ray detector which is sufficiently satisfactory in the S/N ratio, and in turn an X-ray CT apparatus have not been provided, like the conventional X-ray detector as mentioned above.